Photosynthesis of Au/TiO2 nanoparticles for photocatalytic gold recovery from industrial gold-cyanide plating wastewater

A series of Aux/TiO2 nanoparticles (NPs) with different gold loadings (x = 0.1–1.0 wt%) was synthesized by the photodeposition and then employed as photocatalysts to recover precious component from the industrial gold-cyanide plating wastewater. Effects of Au loading, catalyst dosage and types of hole scavenger on the photocatalytic gold recovery were investigated under ultraviolet–visible (UV–Vis) light irradiation at room temperature. It was found that different Au loadings tuned the light absorption capacity of the synthesized photocatalysts and enhanced the photocatalytic activity in comparison with the bare TiO2 NPs. The addition of CH3OH, C2H5OH, C3H8O, and Na2S2O3 as a hole scavenger significantly promoted the photocatalytic activity of the gold recovery, while the H2O2 did not. Among different hole scavengers employed in this work, the CH3OH exhibited the highest capability to promote the photocatalytic gold recovery. In summary, the Au0.5/TiO2 NPs exhibited the best photocatalytic activity to completely recover gold ions within 30 min at the catalyst dosage of 0.5 g/L, light intensity of 3.20 mW/cm2 in the presence of 20 vol% CH3OH as hole scavenger. The photocatalytic activity slightly decreased after the 5th cycle of recovery process, indicating its high reusability.

Recently, it was reported that the TiO 2 /GrSiO 2 exhibited the photocatalytic activity to remove gold ions from gold-cyanide plating solutions higher than the bare TiO 2 6 . The release of free cyanide ions from the stable metal cyanocomplexes can be achieved by an increase in the availability of cyanide for the subsequent oxidation treatment. The presence of hole scavenger such as methanol (CH 3 OH) can promote the deposition of metallic Au NPs on the surface of utilized photocatalysts. The use of ZnS can induce the reduction of the gold-cyanide complexes as well as the reverse oxidation of gold NPs via the photogenerated holes at VB 20 . Approximately 38% of gold was recovered within 120 min via the use of Na 2 SO 3 as the hole scavenger. The ZnO photocatalyst exhibited a high selectivity to recover gold ions from the potassium gold-cyanide wastewater due to its appropriate VB position 21 . The crystalline quality of the ZnO nanopowder affected positively the efficiency of gold recovery. A complete gold recovery can be achieved with 30 min in the presence of 10 vol% CH 3 OH. The interaction between the rGO and TiO 2 in the rGO/TiO 2 composite can alleviate the rate of electron-hole (e − -h + ) recombination and enhance the charge transfer between RGO and TiO 2 structure, thus promoting the photocatalytic gold recovery from the gold-cyanide complex solution 22 . Based on mentioned above, it can be noted the photocatalytic activity of gold recovery depeneds upon various parameters, particularly the photocatalyst type, band position and hole scavenger.
In this work, a series of Au x /TiO 2 was synthesized via the photodeposition using the commercial TiO 2 as based material. Effects of gold loading on morphology and optical property of the obatined Au x /TiO 2 NPs as well as the photocatalytic acitivity for gold recovery from the industrial plating wastwater were explored. The optimum hole scavenger type and catalyst dosage were also determined.

Methods
Property of utilized gold-cyanide plating wastewater. The utilized gold-cyanide plating wastewater was generously offered from the circuit board industry located in Phra Nakhon Si Ayutthaya, Thailand. This solution has a light-green clear color with the pH of 8.4-9.2. It contains high concentration of Au ions (10-15 mg/L) and a trace quantity of Cu, Ni, potassium (K), zinc (Zn) of less than 0.06, 0.46, 202.85 and 2.17 mg/L, respectively.
Photocatalyst preparation and characterization. Gold nanoparticles at different contents (0.1-1.0 wt%) were deposited on the surface of commercial TiO 2 (99.5%, Sigma-Aldrich) by the photodeposition using the chloroauric acid (99.9% HAuCl 4 ·3H 2 O; Sigma-Aldrich) as the chemical precursor. Briefly, approximately 1.2 g of TiO 2 (99.5%, Sigma-Aldrich) were dispersed in the 300 mL of 5 mg/L HAuCl 4 solution in a double wall cylindrical glass reactor. The pH of the HAuCl 4 solution was adjusted to 10 by 0.5 M sodium hydroxide (NaOH; Sigma-Aldrich). The glass reactor was then put on the hotplate stirrer (MSH-300, Biosan) and placed centrally in the UV-protected box. The external cold water was supplied to the jacket of the glass reactor and circulated for the whole experimental time to control the operating temperature (~ 30 °C). A solid-liquid mixture was constantly stirred at the rate of 400 rpm in the absence of light for 30 min to allow a good adsorption of gold ions on the surface of TiO 2 NPs. Afterward, a high-pressure mercury lamp (400 W; RUV 533 BC, Holland) was turned on to generate the UV-visible light at the wavelength of 100-600 nm. The incident light intensity at the reactor was kept constantly at around 3.20 mW/cm 2 . When a complete photodeposition was achieved (~ 1 h), the solid NPs was first separated from a solid-liquid mixture by centrifugal at 11,000 rpm (Eppendorf, 5804R) and washed thoroughly with deionized (DI) water. The ready-to-use Au 0.1 /TiO 2 NPs was obtained after drying at 80 °C for 3 h. The similar procedure was carried out to prepare Au 0.3 /TiO 2 , Au 0.5 /TiO 2 and Au 1.0 /TiO 2 via the use of HAuCl 4 solution at gold ion concentrations of 15, 25 and 50 mg/L, respectively.
The morphology and optical property of all synthesized Au x /TiO 2 NPs as well as the TiO 2 were characterized as following. XRD patterns were taken on a Bruker D2 Phaser diffractometer using Cu Kα X-ray). The anatase fraction and average crystallite size of all photocatalyst NPs were computed via Spurr and Myers equation (Eq. 1) and Debye-Scherrer equation (Eq. 2), respectively 23 .
where x A is the anatase weight fraction, I A is the relative reflection intensities of anatase and I R is the relative reflection intensities of rutile.
where D is the average crystallite size (nm), λ is the wavelength of the X-ray radiation (0.154178 nm), β is the full width at half maximum intensity of the peak and θ is the diffraction angle. Scanning electron microscopy (SEM) was carried out via a JSM-IT500HR and equipped with a JED-2300 energy dispersive X-ray (EDX) spectrometer at landing voltage of 15.0 kV and magnification × 3000. The content of metallic Au NPs on TiO 2 was examined by inductively coupled plasma mass spectrometry (ICP-MS, PerkinElmer, NexION 2000) using hydrochloric acid (37% HCl, Merck) and nitric acid (68% HNO 3 , BDH) at volume ratio of 3:1 as extractant. High-resolution transmission electron microscopy (HRTEM) was taken via a JEM-3100F with an accelerating voltage of 300 kV. XPS data were harvested from Axis Supra + (Kratos, UK) with a Delay Line detector (DLD) and a monochromatic Al Kα (hν = 1,486.6 eV) source. The spectra were calibrated with the C1s peak of adventitious carbon at binding energy of 284.8 eV to minimize the error of binding energies within the range of ± 0.1 eV. The UPS mode of this analysis allowed to estimate the value of Fermi edge energy (E f ) and cutoff edge energy (E c ), which can be used to compute the photocatalytic work function (Φ) as well as the valence band energy (E v ) and conduction band energy (E c ) according to Eqs. (3)-(5) 24-26 . where Φ is the work function of photocatalysts, hv is the excitation source which equals to 21.2 eV for helium (He), E f is the Fermi edge energy, E c is the cutoff edge energy, E V is the valence band energy, E C is the conduction band energy, E a is the electron affinity of TiO 2 NPs (3.9 eV 27 ) and E g is the band gap energy obtained from UV-Vis analysis and Tauc's plot. N 2 adsorption/desorption isotherms were detected using a Multipoint Surface Area Analyzer (Tristar II3020, Micromeritics) via the Brunauer-Emmett-Teller (BET) method. The photoluminescence (PL) data were taken on a Perkin-Elmer LS-55 Luminescence Spectrometer in air at ambient temperature with a 290 nm cut-off filter. The PL signals were collected in the range of 350-550 nm using a standard photomultiplier. UV-Vis absorption spectra were monitored over the wavelength range of 200−800 nm via a Cary300 UV-Vis spectrophotometer (Agilent).
Photocatalytic activity of Au x /TiO 2 for gold recovery. The experimental set up for testing the photocatalytic activity of the synthesized Au x /TiO 2 NPs for gold recovery from gold-cyanide plating wastewater was similar to the previous section. That is, approximately 0.6 g of respective Au x /TiO 2 was dispersed in 300 mL of gold-cyanide plating wastewater in the presence of selected hole scavengers including hydrogen peroxide (H 2 O 2 , BDH Laboratory), sodium thiosulphate (Na 2 S 2 O 3 , KemAus), methanol (CH 3 OH, QReC), ethanol (C 2 H 5 OH, QReC), n-propanol (n-C 3 H 8 O, KemAus), i-propanol (i-C 3 H 8 O, QReC) and glycerol (C 3 H 8 O 3 , KemAus). The solid-liquid mixture was stirred constantly at 300 rpm in dark environment for 30 min to allow a thoroughly dispersion and adsorption of gold complex species on the photocatalyst surface. Then, the system was irradiated by UV-Vis light (100-600 nm) generated by a high-pressure mercury lamp (400 W; RUV 533 BC, Holland). The light intensity was fixed over the whole experimental period at 3.20 mW/cm 2 . The temperature of the reactor was controlled at 28-30 °C by the water circulation at the reactor jacket using the circulating pump (PMD-0311, Sanso, Japan). As the reaction proceeded, approximately 5 mL of solution was taken at particular time and subjected to centrifuge at 11,000 rpm (5804R, Eppendorf) to separate the solid NPs from solution. The remining concentration of gold ions in solution at particular time was measured by flame atomic absorption spectrometry (Flame-AAS, Analyst 200 + flas 400; Perkin-Elmer). The reduction rate of gold ions from the goldcyanide plating wastewater was fitted via the Langmuir-Hinshelwood model according to Eq. (6) 28 .
where C 0 is the initial gold ion concentration (mg/L), C is the gold ion concentration at particular time (mg/L), t is the reaction time (min) and k is the first-order reaction rate constant (min −1 ).

Results and discussion
Morphology and optical property of synthesized Au x /TiO 2 . To shed some light on the crystallite structure of all Au x /TiO 2 photocatalysts, the XRD analysis was first carried out. As depicted in Fig. 1 The presence of deposited Au NPs on the surface TiO 2 was then explored via the SEM-EDX analysis. As displayed in Fig. 2, a uniform distribution of Au NPs was observed for all loadings. The amount of deposited gold Au0.3/TiO2 Au0.5/TiO2 www.nature.com/scientificreports/ examined by both SEM-EDX and ICP was closed to the preset content as summarized in Table 1. The average particle size of deposited Au NPs was then examined using a high-resolution TEM analysis. As clearly shown in Fig. 3, all HRTEM images showed a clear feature of both TiO 2 and Au NPs. The TiO 2 NPs exhibited a spherical shape of anatase crystallites (~ 21 nm) and also an angular shape of rutile crystallites (~ 30 nm). The Au NPs exhibited a pseudo-spherical shape with average particle size in the range of 5.8-8.0 nm (Table 1). According to the particle size distribution, it seems to be that the size of Au NPs was largely dependent on the Au loading Table 1. Morphology and optical property of all synthesized Au x /TiO 2 and the parental TiO 2 NPs. a Estimated from XRD analysis using Spurr and Myers equatio b Estimated from XRD analysis using Debye-Scherrer equation. c Estimated from HRTEM analysis.

Catalysts A/R ratio a
Crystallite size b (nm) Au content (wt%)

Size of Au NPs c (nm)
Textural property

E g (eV) E c (eV) E f (eV) TiO 2 (A) TiO 2 (R) SEM-EDX ICP
BET area (m 2 /g) Total pore volume (cm 3 /g)  To probe the presence of chemical elements as well as the oxidation state of gold on the surface of TiO 2 , the XPS analysis was carried out. As demonstrated in Fig. 4a, the survey XPS showed the intensive peaks of Ti2p and O1s of Ti and O species at binding energy of around 458.3 and 530.2 eV, respectively. The high resolution XPS spectra of Ti2p components in the parental TiO 2 structure exhibited two symmetric spectra at binding energy of 458.3 and 464.1 eV (Fig. 4b), corresponding to the spin-orbital doublet of the Ti2p3/2 and Ti2p1/2 components 29,30 , which indicates the presence of Ti 4+ species in TiO 2 NPs 31 . The Ti2p XPS spectra of Au x /TiO 2 NPs also revealed the symmetric spectra of Ti2p3/2 and Ti2p1/2 components without any shifting of binding energy compared with those of parental TiO 2 NPs. This suggested that the addition of Au NPs on the surface of TiO 2 by the photodeposition cannot enhance the formation of the disorder and/or defective structure of TiO 2 . The Au4f XPS spectra of all Au x /TiO 2 NPs showed the core-level electrons at binding energy of ~ 82.9 and ~ 86.5 eV, respectively (Fig. 4c), assigning to the Au4f7/2 and Au4f5/2 of metallic gold 32,33 . The intensity of core-level features increased linearity as the increasing gold loading. The Au4f spectra of gold ions were not detected, suggesting that the original gold ions in gold chloride precursor were completely reduced to metallic gold via the photodeposition.
Regardless the textural property of all synthesized samples, the N 2 adsorption/desorption isotherms of all synthesized Au x /TiO 2 NPs together with the parental TiO 2 NPs were examined and displayed in Fig. 5 Intensity (a.u.) Binding energy (eV) Au0.1/TiO2 Au0.3/TiO2 Au0.5/TiO2 Au1.0/TiO2 Au0.3/TiO2 Au0.5/TiO2  Table 1. The Au 1.0 /TiO 2 exhibited the BET surface area slightly higher than that of other Au x /TiO 2 NPs as well as the parental TiO 2 , probably due to its well dispersion on the surface of TiO 2 26,35 , which can enhance a high adsorption capacity. The qualitative recombination rate of e − -h + pairs was then determined using the photoluminescence (PL) spectrometer. An intense peak of PL signal indicates a high recombination rate of e − -h + pair. From the plot, the TiO 2 NPs revealed three highly broad PL emission peaks, centered at 422.5, 484.0 and 528.0 nm (Fig. 6), indicating a fast e − -h + recombination. A high-energy spectrum at 422.5 nm is associated from the band edge excitation of TiO 2 , other spectra are attributed to the electron transition by the state of oxygen vacancies and/ or defective structure 36,37 . All Au x /TiO 2 NPs showed lower intense PL spectra than those of TiO 2 NPs, indicating that the presence of Au NPs can suppress the rate of e − -h + recombination. Among all Au x /TiO 2 NPs, the intensity of PL spectra decreased as the increase of gold loading (inset of Fig. 6). This can be explained in terms of the charge separation at Schottky junction and the localized surface plasmon resonance (LSPR) phenomenon of Au NPs 38 . High deposited gold may function as the electron sink, allowing the migration and movement of  www.nature.com/scientificreports/ electrons from the surface to the bulk which consequently promote the e − -h + separation as well as suppress the e − -h + recombination 39 .
To further explore the optical property of all synthesized samples, the light absorption capacity of all synthesized Au x /TiO 2 NPs as well as the parental TiO 2 was then analyzed. As illustrated in Fig. 7a, the parental TiO 2 NPs exhibited a strong absorbance during the ultraviolet (UV) light regime (λ < 400 nm) due to its visible light inertness in nature. All Au x /TiO 2 NPs exhibited strong absorption spectrum in the visible light region (λ > 400 nm) with the broad absorption features ~ 540-570 nm, assigning to the LSPR behavior of deposited Au NPs 33 . The intensity of LSPR peaks increased as the increase of nominal gold loading up to 0.5 wt% and dropped slightly afterward (inset of Fig. 7a). A high light absorption might enhance the harvesting of incident light and also promote the photocatalytic activity. Slight red-shift in the plasmon position was observed with the increased nominal gold loading due to the size increase of Au NPs 40 . Tauc plots or plots of (αhν) 1/n versus photon energy (E) computed from the UV-Vis absorption spectra (Fig. 7a) are shown in Fig. 7b. The E g of each sample can be obtained by extrapolation the linear portion of this plot to intercept the x-axis. As summarized in Table 1, the E g energy of TiO 2 decreased importantly from 3.35 to 3.22 eV with addition of Au NPs. Figure 8a illustrates the intensity-kinetic energy plots of commercial TiO 2 and all synthesized Au x /TiO 2 NPs. Values of E f and E c taken respectively from the x-axis interception at high-and low binding energies of this plot are summarized in Table 1. Both obtained energy values were then used to compute the Φ as well as the E V and E C according to Eqs. (3)-(5). It was obtained that the Φ of TiO 2 , Au 0.1 /TiO 2 , Au 0.3 /TiO 2 , Au 0.5 /TiO 2 and Au 1.0 / TiO 2 NPs could be determined to be 6.38, 6.23, 6.20, 6.14 and 6.25 eV, respectively. Figure 8b represents the briefly sketch of band position of all synthesized Au x /TiO 2 and commercial TiO 2 NPs. It is noteworthy that both synthesized Au x /TiO 2 and the parental TiO 2 NPs had a more negative conduction band level than the reduction potential of [Au(CN) 2 ] − , indicating their ability to act as the photocatalyst for gold reduction from the goldcyanide plating wastewater. www.nature.com/scientificreports/ gold recovery from the gold-cyanide plating wastewater at the photocatalyst dosage of 2 g/L and light intensity of 3.20 mW/cm 2 in the absence of hole scavenger. As shown in Fig. 9, the concentrations of gold ions did not change at particular time in the presence of TiO 2 NPs, while it decreased significantly in the presence of Au x / TiO 2 NPs. Among all Au x /TiO 2 samples, the Au 0.5 /TiO 2 NPs can achieve a high reduction of gold ions from the plating wastewater. This indicated that the Au 0.5 /TiO 2 exhibited the highest photocatalytic activity for gold recovery from the plating wastewater. Its apparent rate constant estimated from the Langmuir-Hinshelwood model (Eq. 6) was found to be at 0.0190 min −1 , which was higher than those of TiO 2 , Au 0.1 /TiO 2 , Au 0.3 /TiO 2 , and Au 1.0 / TiO 2 for 38.8, 2.1, 1.4 and 1.6-fold, respectively. Taking into account the morphology and optical property of all photocatalysts, it can be seen that the variation trend of the photocatalytic activity of all photocatalysts did not correlate with the content or size of deposited gold NPs, height of PL spectra as well as the total pore volume (Table 1). However, it slightly changed corresponding to the variation of the E g and height of LSPR peak. That is, the Au 0.5 /TiO 2 NPs exhibited the lowest E g of 3.22 eV and the highest height of LSPR peak and also depicted the highest photocatalytic gold recovery from the gold-cyanide plating wastewater. Interestingly, although the Au 1.0 /TiO 2 NPs showed the lowest PL spectra, comparable E g to Au 0.3 /TiO 2 and comparable LSPR peak height to Au 0.5 /TiO 2 , it showed lower the photocatalytic activity for gold recovery than both Au 0.3 /TiO 2 and Au 0.5 /TiO 2 NPs. This might be due to its high deposited gold content that allowed a freely transfer of excited electron along the bulk phase of gold structure and thus in turn slowdown the photocatalytic activity due to their function as an electron sink 39 . Another possible reason might be due to the synergetic effect of electron transfer between the anatase, rutile and metallic gold 33 . That is, a high gold content can randomly deposit on the anatase-rutile interface and also on the isolated anatase or rutile crystallite. The former gave the positive effect to the photocatalytic of Au x /TiO 2 NPs, while the latter gave the negative effect 33 . A high gold content probably induced a high proportion of Au/rutile, which consequently suppressed the photocatalytic activity of Au x /TiO 2 NPs. Besides, a high gold loading might form the shadowing behavior, thus preventing the absorption of incident light of the photocatalyst NPs 33,41 . www.nature.com/scientificreports/ Effect of hole scavengers. Since the photocatalytic recovery of gold from gold-cyanide plating wastewater is impelled by the photogenerated e − , a rapid recombination of photogenerated h + and e − may suppress the photocatalytic activity. To extend the lifetime of e − -h + pairs, the removal of photogenerated h + from the photocatalyst was carried out by introduction of sacrificial hole scavengers to the plating wastewater. In this part, three types of hole scavengers including H 2 O 2 , Na 2 S 2 O 3 and CH 3 OH were first employed at identical quantity of 20 vol%. The variations of gold recovery via Au 0.5 /TiO 2 photocatalyst at dosage of 2 g/L and light intensity of 3.20 mW/cm 2 in the presence of different hole scavengers were depicted in Fig. 10. The addition of H 2 O 2 did not promote the photocatalytic gold recovery via the utilized photocatalyst, while the addition of Na 2 S 2 O 3 and CH 3 OH encouraged the photocatalytic rate of gold recovery. The apparent rate constants of the system using Na 2 S 2 O 3 and CH 3 OH were 0.0688 min −1 and 0.1332 min −1 which were higher than that in the absence of hole scavenger of 3.6-and 7.0-fold, respectively. The different photocatalytic activity of each hole scavenger might be attributed to the formation of different ionic species when the hole scavengers reacted with the photogenerated hole 42 . That is, the active form of hole scavenger in the presence of H 2 O 2 was the OH − , which is coming from the reaction of H 2 O 2 via the incident light or the photogenerated e − according to reactions (R4)-(R6) 43,44 . Based on these reactions, the use of H 2 O 2 as the hole scavenger consumes the photogenerated e − competitively with the photocatalytic reduction of gold ions to metallic Au NPs, which may suppress the photocatalytic gold recovery of Au x /TiO 2 NPs. Besides, the lack of photocatalytic gold recovery in the presence of H 2 O 2 might be originated from its fast decomposition to H 2 O and O 2 , which can be experimentally observed immediately after the addition of this hole scavenger. A low photocatalytic activity in the presence of H 2 O 2 as the hole scavenger was also observed for the H 2 production via the TiO 2 nanotubes 45 .  46 . For comparison, when CH 3 OH reacted with the photogenerated holes in the Au 0.5 /TiO 2 NPs, the methoxy radicals (CH 3 O·) may form according to reaction (R8) 37,42 . These generated radicals can effectively supply electrons to the catalyst surface due to its low standard reduction potential (reaction (R9)) 47,48 , thus promoting the reduction of the adsorbed gold-cyanide species.

Photocatalytic activity of Au
The effect of alcohol types including CH 3 OH, C 2 H 5 OH, n-C 3 H 8 O, i-C 3 H 8 O and C 3 H 8 O 3 on the photocatalytic gold recovery via Au 0.5 /TiO 2 photocatalyst was further explored at identical quantity of 20 vol% using the photocatalyst dosage of 2 g/L and light intensity of 3.20 mW/cm 2 . As shown in Fig. 11, the positive effect of hole scavenger on the photocatalytic gold recovery can be ranked as the order of CH 3 OH > C 3 H 8 O > C 2 H 5 OH > n-C 3 H 8 O > i-C 3 H 8 O. This is probably due to the effect of their different oxidation potentials. That is, the driving force of hole scavenging is dictated by the oxidation potential of the hole scavenger molecules 45 . The chemicals with low oxidation potential thermodynamically exhibits a rapid hole scavenging and vice versa 37,45 . Based on the obtained results, this fact is mostly conformed for the CH 3 OH (0.016 V/NHE), C 2 H 5 OH (0.084 V/NHE), n-C 3 H 8 O (0.100 V/NHE) and i-C 3 H 8 O (0.105 V/NHE) 37 , except the C 3 H 8 O (0.004 V/ NHE). Although CH 3 OH has the oxidation potential four times higher than that of C 3 H 8 O 8 , it exhibited a higher positive effect on the photocatalytic gold recovery. This might be due to the electron dissemination of the generated CH 3 O · radicals according to reaction (R9), which then reduce the adsorbed gold-cyanide species to metallic Au NPs. Interestingly, the system with i-C 3 H 8 O exhibited an extremely low gold reduction during the first 90 min although its oxidation potential is slightly lower than that of n-C 3 H 8 O. This might be attributed the effect of the molecular steric hindrance of this branched alcohol. That is, a high quantity of i-C 3 H 8 O at the early reaction period may obstruct each other to react with the photogenerated h + , thus lower the photocatalytic activity. However, as the time proceeded, part of this alcohol was consumed. Therefore, the photocatalytic activity increased due to the lessening of the steric hindrance effect. Based on the obtained results and literature 6,37,42,49 , the mechanism of photocatalytic gold recovery from the gold-cyanide plating wastewater on the Au/TiO 2 NPs in the presence of CH 3 OH was roughly sketched as illustrated in Fig. 12.
Effect of catalyst dosage. The effect of photocatalyst dosage (0.5-2.0 g/L) on the photocatalytic gold recovery was then examined at light intensity of 3.20 mW/cm 2 in the presence of 20 vol% CH 3 OH. As displayed in Fig. 13, approximately 93% of gold ions was recovered within 15 min via the use of 0.5 g/L of Au 0.5 /TiO 2 NPs, while greater than 98% was recovered via the same photocatalyst at the dosage of 1.0-2.0 g/L at the same time. A low photocatalytic activity to recover gold at low photocatalyst dosage might be due to the limitation of active site www.nature.com/scientificreports/ to proceed the photoreaction. Nevertheless, too high photocatalyst dosage also exhibited a low photocatalytic activity due to the light scattering effect as described by Beer-Lambert Law and also the light attenuation due to the self-shading behavior 50,51 . Thus, it can be noted that the optimum dosage of Au 0.5 /TiO 2 NPs for the photocatalytic gold recovery from the spent gold-cyanide plating wastewater was 1.0 g/L. Table 2 summarizes the comparative photocatalytic gold recovery from the gold-cyanide containing solution between this work and previous works. It is worth noting that the Au 0.5 /TiO 2 photocatalyst synthesized in  www.nature.com/scientificreports/ this work was on par with other previous works. It exhibited a higher photocatalytic activity than TiO 2 52 , TiO 2 / GrSiO 2 6 and ZnS 20 and comparable activity with ZnO 21 . With respect to rGO/TiO 2 , it is difficult to conclude that which photocatalyst type is better between rGO/TiO 2 and Au 0.5 /TiO 2 due to their high different initial gold ions concentration (~ 13.7-fold). Figure 14 displays the variation of repetitive gold recovery from the gold-cyanide plating wastewater via the Au 0.5 /TiO 2 photocatalyst at loading of 1.0 g/L and light intensity of 3.20 mW/cm 2 in the presence of 20 vol% CH 3 OH as the hole scavenger. After each photocatalytic experiment, the utilized photocatalyst was washed delicately with DI water and dried in air at 80 °C for 3 h and then subjected to the next photocatalytic run. It can be seen that a fast decrease of gold ions was observed for all repetitive runs, indicating a high resuability of the synthesized Au/TiO 2 NPs. The normalized concentration of remianing gold ions were around 0.02, 0.021, 0.027, 0.043 and 0.061 after the 1st, 2nd, 3rd, 4th and 5th run, respectively. The purple color of the fresh Au 0.5 /TiO 2 NPs intensified with the increasing repetive runs, due to the increasing deposited Au NPs on photocatalyst surface 33,53 .
The crystallite structure of Au 0.5 /TiO 2 NPs after all repetitive experiments was also analyzed as shown in Fig. 15. The XRD pattern of the Au 0.5 /TiO 2 NPs after the 1st, 3rd and 5th run were still demonstrated the main characteristic peaks of anatase-and rutile phase as described above. Besides, they exhibited the diffraction peaks of Au NPs at 2θ of 38.  www.nature.com/scientificreports/ height was observed after a high repetitive run. By SEM-EDX analysis, the gold contents on the Au 0.5 /TiO 2 after the 1st, 3rd and 5th run were around 2.18, 4.56 and 7.53 wt%, respectively. Despite the repetitive runs of gold recovery, an intensely uniform dispersion of Au NPs on the TiO 2 surface was observed as illustrated in Fig. 16. As proposed by Grieken et al. 6 , the deposited Au NPs can be separated from the TiO 2 based material by the selective dissolution via (i) the aqua regia to leach metallic Au NPs, yielding the tetrachloroauric acid (HAuCl 4 ) or (ii) hydrofluoric acid (HF) to leach the based catalyst, getting a brown gold power. To achieve the objective related to the gold recovery from industrial gold-cyanide plating wastewater, the selective dissolution of deposited Au NPs from the TiO 2 was carried out using aqua regia as the extractant. By ICP analysis, the obtained solution contained gold ions of around 57, 140 and 224 mg/L, which were related to 2.19, 5.14 and 7.96 wt% Au on TiO 2 after the 1st, 3rd and 5th run, respectively, consistent with those analyzed by SEM-EDX.

Conclusion
A set of Au x /TiO 2 NPs with different Au loadings was prepared via the photodeposition for photocatalytic gold recovery from industrial gold-cyanide plating wastewater. The deposited Au NPs in the investigated range of 0.1-1.0 wt% exhibited the insignificant effect on the A/R ratio, crystalline size of TiO 2 , surface area, but it affected importantly the size of deposited Au NPs as well as the light absorption capacity. Among all synthesized photocatalysts, the Au 0.5 /TiO 2 NPs exhibited the best photocatalytic activity to recover gold from the industrial gold-cyanide plating wastewater. The addition of Na 2 S 2 O 3 and C 1 -C 3 alcohol as the hole scavengers promoted the photocatalytic gold recovery over the Au 0.5 /TiO 2 NPs, while the H 2 O 2 did not. Among all employed alcohols, the CH 3 OH exhibited the highest efficiency to promote the photocatalytic gold recovery. A complete recovery of gold ions can be achieved within 30 min at the photocatalyst dosage of 0.5 g/L, light intensity of 3.20 mW/cm 2 in the presence of 20 vol% CH 3 OH as hole scavenger. The synthesized Au 0.5 /TiO 2 NPs exhibited higher photocatalytic activity for gold recovery from the gold-cyanide containing solution than some photocatalysts in literature 6,20,52 . A very slight decrease of the photocatalytic gold recovery was observed after the 5th run, indicating its high reusability and high ability to accumulate the metallic Au NPs on its surface. The separation of deposited Au NPs from the used Au/TiO 2 photocatalyst can be carried out by the selective dissolution using the chemical extractant. Besides, the repetitively used Au/TiO 2 NPs are possibly used as the photocatalysts for other photocatalytic application such as H 2 production 26,53-56 , but more characterizations and photocatalytic activity tests are required.

Data availability
All data generated or analyzed during this study are included in this published article.